What is the risk of becoming schizophrenic if you smoke marijuana?

Research shows that teens who use marijuana heavily have physical anomalies in their brains years later, and those differences from non-smokers’ brains look like the physical anomalies in schizophrenics. So, we ask: Does marijuana make you crazy? Or does being crazy lead to heavy marijuana use? (Getty Images)

Most people you know who smoke marijuana, even the heaviest users, will not become schizophrenic, nor go psychotic. Notice I did not say, “Marijuana use will not cause psychosis in the vast majority of users,” because the causal link between marijuana use and such diseases has not been made. (More on that below.)

“The vast majority of people that smoke cannabis come to no harm,” says one of the world’s foremost researchers into cannabis use and mental illness: Professor Robin Murray of the Institute of Psychiatry, King’s College London. Murray made these statements in a YouTube video.

“It’s just like alcohol. The vast majority of people who drink, come to no harm, even people who drink heavily come to no harm and it’s a bit similar with cannabis. But, equally for alcohol, a small proportion become dependent. (Drinkers) develop alcoholism and develop all sorts of physical problems; and for cannabis, a small proportion go psychotic.” (A fuller statement is in a video and a longer quote below.)

But why is marijuana associated with schizophrenia?

That’s what researchers are trying to figure out, and there have been three key studies, recently published, that point in two different directions, which is why I chose to start this story as I did. Rather than blast out headlines that contribute to the whipsaw, head-jerking of “Yes it does!” and “No it doesn’t!” let’s start with what none of this research does.

None of the research on cannabis and schizophrenia establishes or disproves a causal link. No research has proven that ingestion of the chemicals in marijuana causes physical damage that results in clinical psychosis (as opposed to just thinking someone is stupid or nuts).

What two of the research articles I’ll bring up do establish is a correlation between marijuana use and the presence of brain structure anomalies found in people with schizophrenia. The third research finding establishes that there is no strong correlation between cannabis use and schizophrenia.

There was no significant difference in morbid risk for schizophrenia between relatives of the patients who use or do not use cannabis. … The results of the current study suggest that having an increased familial morbid risk for schizophrenia may be the underlying basis for schizophrenia in cannabis users and not cannabis use by itself.

This Harvard study is a data study. The researchers collected already published data from wide sources, crunched the numbers and found that there was a much stronger correlation between family dynamics and schizophrenia than a correlation between pot and this psychosis.

Physical changes in the brain

The other two studies are pretty robust science in that they not only crunched data already published elsewhere but also physically examined patients with MRIs. Both of these studies found similar abnormal brain structures in heavy pot smokers and people with schizophrenia.

Abnormalities in hippocampal morphology are characteristic of schizophrenia and have also been reported in chronic cannabis users. … In this study … Hippocampal shape changes were observed in each group relative to controls, with the greatest degree of alterations … in cannabis-using schizophrenia patients. These alterations were associated with cannabis use patterns and psychotic symptoms.

This study found that people reporting heavy pot smoking when they were teenagers had abnormal brain parts years later — mostly the thalamus. The abnormal brain parts also looked like those in people with schizophrenia. Another key finding was that people with these abnormal shapes performed poorly on “working memory” tests.

Scarily, the researchers found that the type of abnormal shape in pot smokers was more pronounced in schizophrenics who were also pot smokers.

We called up Matthew Smith, lead scientist on this study and an assistant research professor at Northwestern University Feinberg School of Medicine, for a plain-language account of his study. Here’s what he said:

… our data shows that the more marijuana abnormality that you see in that area of the brain, the poorer (those people) did on the memory tests … Just the consistency across the two groups (heavy users and people with schizophrenia) is pretty striking. Now, as far as causal. Our data is at “one point in time” so we cannot just say, ‘Marijuana causes these brain abnormalities.’

But, there’s two ways to interpret it: Either they cause it or these brain abnormalities were preexisting to any amount of marijuana use. So, that’s the chicken or egg thing.

That said, the younger the participants were when they started using marijuana on a daily basis the more severe the abnormalities are apparent; and again … we found that in not only the control marijuana use group but also the (schizophrenia) group. So we have these striking similarities in the way the brain looks, the way the brain looks as it relates to performance, poor performance; but also the younger they started using, the more abnormal things looked.

Our data can’t confirm causation, however, especially the correlation we found between the age of onset of daily marijuana use and the more brain abnormality (we saw), that correlation itself, it’s not causal but it is supportive of what we would call a “causal hypothesis.” It would say, “Hey, this could be causal and we should really look at this more.”

Will this affect legalization? Or, who cares?

Kids brains, especially teenagers brains, are so screwed up they definitely don’t need more drugs. Hell, surging hormones plus daily caffeine and sugar diets would be enough to make a grown man/woman psychotic.

So, it’s clear kids shouldn’t smoke pot. The big question is whether legalization will cause more kids to use marijuana than they do now. Surveys make it clear that kids already report marijuana easy to get. So, you know how much more easy can it get? The second question is in a legal market where pot use doesn’t have to be hidden, will kids get more information they’ll trust about the negative effects of marijuana and, like smoking, curb their use (at least a little).

The other thing to remember is that after decades of clear negative research results about alcohol use … no one is likely to get far with a new temperance movement on that front.

So, if this risk of schizophrenia is proven, will that stop the forward progress of legalization?

Just a guess here, but I’m gonna say, Nope. Voters don’t appear to think that marijuana is good for you … they appear to be voting it in because it isn’t any worse, at least, than alcohol and the war on drugs causes more social and personal damage than the drug itself.

Legalization is not creating marijuana use nor a market for it. After decades of prohibition, use has even slightly increased over that time. So it’s not like folks are going to suddenly stop using because of some bad news on the research front. And until people decide to stop using cannabis, we’ll be right where we are with a raging black market and prisons full of non-violent offenders.

… after all it’s taken decades to reduce smoking, and alcohol consumption seems totally unimpaired by the litany of body and brain killing side effects.

It is difficult to look at the relationship between environmental and genetic factors, but we have examined that question in relation to cannabis. We now know that there is an interaction between the catechol-O-methyl transferase gene (COMT), which some regard as a susceptibility gene for schizophrenia, and cannabis consumption. People with the Val/Val variant of COMT are much more likely to develop psychosis if they abuse cannabis in their adolescence, but there is no evidence that people with the Val/Val genotype of COMT actually take more cannabis than the rest of the population. So it’s not that the cannabis consumption is a manifestation of predisposition to schizophrenia. Rather, cannabis consumption interacts with genetic susceptibility to schizophrenia. The result is that the majority of the population can abuse as much cannabis as they like and don’t come to much harm. But a vulnerable minority, about 25 percent of the population, is prone to psychotic reactions if they take regular cannabis (Caspi et al., 2005).

Here the man is in plain English:

A history of marijuana:

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A scene in Bridewell Prison, where the harlot Moll Hackabout is sentenced to beat hemp, on Plate 4 of 'The Harlot's Progress' by William Hogarth, 1732. An engraving by S. Davenport after Hogarth.
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A scene in Bridewell Prison, where the harlot Moll Hackabout is sentenced to beat hemp, on Plate 4 of 'The Harlot's Progress' by William Hogarth, 1732. An engraving by S. Davenport after Hogarth.
Photo By ... more

Tax stamp for a producer of hemp, which served mostly to make producers and sellers of hemp and marijuana products register with the government. If you got caught with the drug and no tax stamp … well, they’d bust ya. (Wikimedia Commons) less

Tax stamp for a producer of hemp, which served mostly to make producers and sellers of hemp and marijuana products register with the government. If you got caught with the drug and no tax stamp … well, ... more

A teenage girl smokes marijuana in New York in June 1953.
Photo By Keystone Features/Getty Images

A teenage girl smokes marijuana in New York in June 1953.
Photo By Keystone Features/Getty Images

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Federal Bureau of Narcotics public service announcement used in the late 1930s and 1940s. (Wikimedia Commons)

Federal Bureau of Narcotics public service announcement used in the late 1930s and 1940s. (Wikimedia Commons)

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Greek dancing sensation Lili Berde performs her erotic number 'The Marijuana,' an act so controversial that it has been banned on British television, in May 1956.
Photo By Juliette Lasserre/Getty Images

Greek dancing sensation Lili Berde performs her erotic number 'The Marijuana,' an act so controversial that it has been banned on British television, in May 1956.
Photo By Juliette Lasserre/Getty Images

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Women dress their hemp crop before weaving the plant into linen in August 1958.
Photo By Keystone Features/Getty Images

Women dress their hemp crop before weaving the plant into linen in August 1958.
Photo By Keystone Features/Getty Images

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A Hippie promotes marijuana legalization outside the courtroom in Chichester, England, where Mick Jagger and Keith Richards of The Rolling Stones were facing drugs charges on May 10, 1967.
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A Hippie promotes marijuana legalization outside the courtroom in Chichester, England, where Mick Jagger and Keith Richards of The Rolling Stones were facing drugs charges on May 10, 1967.
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A group of Hippies talk and smoke marijuana outside their camp at the Isle of Wight pop festival on Aug. 1, 1969.
Photo By Evening Standard/Getty Images

A group of Hippies talk and smoke marijuana outside their camp at the Isle of Wight pop festival on Aug. 1, 1969.
Photo By Evening Standard/Getty Images

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A poster with a quotation in favor of marijuana use by poet Allen Ginsberg hangs on a wall on Sept. 25, 1969.
Photo By Sydney O'Meara/Getty Images

A poster with a quotation in favor of marijuana use by poet Allen Ginsberg hangs on a wall on Sept. 25, 1969.
Photo By Sydney O'Meara/Getty Images

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American pediatrician, psychiatrist and pacifist Dr. Benjamin Spock addresses an audience during his election campaign as candidate for the American presidency in 1972, running against George McGovern and Richard Nixon. His platform included withdrawal of all American troops from everywhere, legalized abortion and marijuana, and amnesty for draft dodgers.
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American pediatrician, psychiatrist and pacifist Dr. Benjamin Spock addresses an audience during his election campaign as candidate for the American presidency in 1972, running against George McGovern and ... more

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A young couple share a marijuana joint on August 1, 1980 at Notting Hill Carnival, west London.
Photo By Evening Standard/Getty Images

A young couple share a marijuana joint on August 1, 1980 at Notting Hill Carnival, west London.
Photo By Evening Standard/Getty Images

Tennis star Jennifer Capriati is shown in a Coral Gables (Fla.) Police Department photo after her arrest on May 16, 1994, for possession of marijuana.
Photo By CORAL GABLES POLICE DEPT/AFP/Getty Images

Tennis star Jennifer Capriati is shown in a Coral Gables (Fla.) Police Department photo after her arrest on May 16, 1994, for possession of marijuana.
Photo By CORAL GABLES POLICE DEPT/AFP/Getty Images

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American rapper Snoop Dogg is escorted in handcuffs by two police officers following his arrest on charges of suspicion of possession of marijuana, circa 1995.
Photo By Hulton Archive/Getty Images

American rapper Snoop Dogg is escorted in handcuffs by two police officers following his arrest on charges of suspicion of possession of marijuana, circa 1995.
Photo By Hulton Archive/Getty Images

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Police constable Denver Sas smokes a water pipe known as a "hubbly bubbly" filled with marijuana on May 11, 1995 before the start of a protest march to legalize the substance in Cape Town, South Africa.
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Police constable Denver Sas smokes a water pipe known as a "hubbly bubbly" filled with marijuana on May 11, 1995 before the start of a protest march to legalize the substance in Cape Town, South ... more

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Members of the "Mars Work Force" anti-narcotics special brigade burn marijuana on Aug. 14, 1995 in Sinaloa State in western Mexico.
Photo By MATIAS RECART/AFP/Getty Images

Members of the "Mars Work Force" anti-narcotics special brigade burn marijuana on Aug. 14, 1995 in Sinaloa State in western Mexico.
Photo By MATIAS RECART/AFP/Getty Images

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An unidentified Canabis Buyers' Club customer (right) lights up a marijuana cigarette for Dennis Person (behind bar), the establishment's director, on May 2, 1996 in San Francisco. Buyers suffering from AIDS, cancer or conditions with symptoms that marijuana is know to alleviate can purchase the outlawed drug with a doctor's note. In the foreground is a bowl of marijuana and marijuana brownies.
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An unidentified Canabis Buyers' Club customer (right) lights up a marijuana cigarette for Dennis Person (behind bar), the establishment's director, on May 2, 1996 in San Francisco. Buyers suffering from AIDS, ... more

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Cheryl Miller from Silverton, N.J., severely disabled by Multiple Sclerosis, is given marijuana to chew by her husband Jim as they participate in a protest against anti-medicinal marijuana legislation, in the doorway of the office of U.S. Rep. Jim Rogan, R-CA, on March 30, 1998, in Washington, D.C. Rogan's office was targeted because he voted for favorable legislation in 1995, citing that a relative found relief by using the drug, and then voted for a House resolution proclaiming opposition to medicinal use of marijuana. Miller was arrested for marijuana possession.
Photo By JOYCE NALTCHAYAN/AFP/Getty Images less

Cheryl Miller from Silverton, N.J., severely disabled by Multiple Sclerosis, is given marijuana to chew by her husband Jim as they participate in a protest against anti-medicinal marijuana legislation, in the ... more

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A Hong Kong woman looks at the display window of British-based cosmetic outlet The Body Shop decorated with a cut-out picture of a cannabis leaf on November 2, 1998. Hong Kong narcotics bureau investigated a range of hemp-based products by the cosmetics company to test whether they contravene drug laws.
Photo By MANUEL CENETA/AFP/Getty Images less

A Hong Kong woman looks at the display window of British-based cosmetic outlet The Body Shop decorated with a cut-out picture of a cannabis leaf on November 2, 1998. Hong Kong narcotics bureau investigated a ... more

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Mother-of-five Sonia Webb campaigns for more relaxed drug laws on the "Cannabus" carrying a 12-metre-long replica of a marijuana joint outside the New South Wales State Parliament at the start of the Drug Summit in Sydney on May 17, 1999.
Photo By TORSTEN BLACKWOOD/AFP/Getty Images less

Mother-of-five Sonia Webb campaigns for more relaxed drug laws on the "Cannabus" carrying a 12-metre-long replica of a marijuana joint outside the New South Wales State Parliament at the start of the Drug ... more

Aaron Sorkin, creator of "The West Wing," looks at his attorney, Steve Sitkoff, during his arraignment May 2, 2001 at the Superior Court in Burbank, Calif., on charges of possession of illegal hallucinogenic mushrooms, rock cocaine and marijuana. Sorkin was arrested April 15, 2001 at Burbank Airport after security officers found a small bag in his carry-on luggage containing paper bundles suspected of containing drugs.
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Aaron Sorkin, creator of "The West Wing," looks at his attorney, Steve Sitkoff, during his arraignment May 2, 2001 at the Superior Court in Burbank, Calif., on charges of possession of illegal hallucinogenic ... more

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Tania Gladstone tries out a hemp pretzel as Sintra Howell looks on at a stand promoting hemp products December 4, 2001 in New York City. The D.E.A. recently banned all hemp food products that contain THC, the active ingredient in marijuana.
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Tania Gladstone tries out a hemp pretzel as Sintra Howell looks on at a stand promoting hemp products December 4, 2001 in New York City. The D.E.A. recently banned all hemp food products that contain THC, the ... more

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An assistant studies marijuana/cannabis leaves in the Maripharma Laboratory February 15, 2002 in Rotterdam, The Netherlands. The Dutch government is the first in the world to officially approve the cultivation and sale of cannabis products to pharmacies for medical purposes.
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An assistant studies marijuana/cannabis leaves in the Maripharma Laboratory February 15, 2002 in Rotterdam, The Netherlands. The Dutch government is the first in the world to officially approve the cultivation ... more

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The norml.org web site posts an ad created by the National Organization for the Reform of Marijuana Laws (NORML) featuring New York Mayor Michael Bloomberg. Bloomberg found himself the poster boy for an ad campaign to legalize marijuana in a full-page spread in the New York Times April 9, 2002.
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The norml.org web site posts an ad created by the National Organization for the Reform of Marijuana Laws (NORML) featuring New York Mayor Michael Bloomberg. Bloomberg found himself the poster boy for an ad ... more

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David Crosby, of Crosby, Stills and Nash, performs May 3, 2003 during the Music Midtown concert in Atlanta, Georgia. Crosby was arrested on marijuana and guns possession charges March 6, 2004 in New York City.
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David Crosby, of Crosby, Stills and Nash, performs May 3, 2003 during the Music Midtown concert in Atlanta, Georgia. Crosby was arrested on marijuana and guns possession charges March 6, 2004 in New York ... more

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Seattle's Hempfest opens on August 21, 2004. The event is billed as the world's largest drug-policy reform rally.
Photo By Ron Wurzer/Getty Images

Seattle's Hempfest opens on August 21, 2004. The event is billed as the world's largest drug-policy reform rally.
Photo By Ron Wurzer/Getty Images

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Proponents of medical marijuana hold signs outside of the U.S. Supreme Court November 29, 2004 in Washington, D.C. The court heard arguments over whether the Controlled Substance Act of 1970 is unconstitutional as it applies to the right to cultivate and possess cannabis to treat medical conditions as recommended by a medical doctor. The court upheld the federal government's right to ban the drug.
Photo By Mannie Garcia/Getty Images less

Proponents of medical marijuana hold signs outside of the U.S. Supreme Court November 29, 2004 in Washington, D.C. The court heard arguments over whether the Controlled Substance Act of 1970 is ... more

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Actor John Kassir , Andy Fickman, director and Matt Blank, CEO and chairman of Showtime pose at the premiere of Showtime's "Reefer Madness" held at the regent Showcase Cinemas on April 5, 2005 in Hollywood, Calif.
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Actor John Kassir , Andy Fickman, director and Matt Blank, CEO and chairman of Showtime pose at the premiere of Showtime's "Reefer Madness" held at the regent Showcase Cinemas on April 5, 2005 in Hollywood, ... more

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In this handout from Drug Enforcement Administration, cross-border tunnel between between the Lynden, WA and Aldergrove, B.C., Canada, is seen July 21, 2005. So-called "B.C. Bud" is commonly smuggled across the border.
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In this handout from Drug Enforcement Administration, cross-border tunnel between between the Lynden, WA and Aldergrove, B.C., Canada, is seen July 21, 2005. So-called "B.C. Bud" is commonly smuggled across ... more

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"Prince of Pot" Marc Emery addresses a crowd of four hundred that attended an anti-extradition rally held for him in front of the U.S. Consulate on September 10, 2005 in Vancouver, B.C., Canada. Emery, leader of the British Columbia Marijuana Party, faced extradition for selling marijuana seeds on the internet.
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"Prince of Pot" Marc Emery addresses a crowd of four hundred that attended an anti-extradition rally held for him in front of the U.S. Consulate on September 10, 2005 in Vancouver, B.C., Canada. Emery, leader ... more

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Alternative Herbal Health Services worker Jason Beck packages medical marijuana April 24, 2006 in San Francisco, after the Food and Drug Administration issued a statement declaring that there is no scientific evidence supporting use of the drug for medical treatment.
Photo By Justin Sullivan/Getty Images less

Alternative Herbal Health Services worker Jason Beck packages medical marijuana April 24, 2006 in San Francisco, after the Food and Drug Administration issued a statement declaring that there is no scientific ... more